Following microfabrication of a circuit pattern of a semiconductor device in recent years, it is required to increase a numerical aperture of a projection lens optical system in an exposure apparatus. The higher the numerical aperture, the shallower the incident angle of pattern diffracted light onto a wafer surface is. That is, the light is made incident at an angle closely parallel to the wafer surface. In imaging at such a shallow incident angle, deterioration of an image contrast by p-polarization, namely polarization in a parallel direction to the plane as defined by a locus of two or more incident light beams becomes remarkable. For that reason, it is important to introduce only s-polarization into the wafer surface without including a p-polarization component. In this way, in fine pattern imaging utilizing a technology for increasing the numerical aperture, it is extremely important to control the polarization of diffracted light for the purpose of enhancing the contrast.
In consequence, birefringence greatly influences controllability of the polarization and deteriorates an imaging characteristic, and therefore, the higher the numerical aperture of an exposure apparatus, the lower the birefringence is required for an optical material to be used therein. On the other hand, in an optical system of the exposure apparatus, an optical member composed of plural synthetic silica glasses and other materials is used. For that reason, the birefringence related to the imaging characteristic on an actual wafer surface is correctly corresponding to one obtained by integrating birefringence of all of optical members through which diffracted light generated from a reticle passes to the wafer (this birefringence will be hereinafter referred to as “optical path integrated birefringence”). In order to decrease this optical path integrated birefringence, besides (1) a method of reducing the birefringence of an individual optical member contained in the same optical system to a considerable extent, there is also (2) a method of reducing the birefringence through compensation utilizing a combination with a fast axis direction in plural optical members contained in the same optical system.
As the foregoing method (1), in order to reduce the birefringence of a synthetic silica glass which is used for the individual optical member, it would be better to remove a residual stress in the synthetic silica glass. In a manufacturing step, it is known to be effective to perform an appropriate cooling treatment on a transparent glass body molded so as to have appropriate size and shape. Examples of the appropriate cooling treatment include a method in which not only for the purpose of releasing a residual stress in the synthetic silica glass, the synthetic silica glass is held at a high temperature for a sufficiently long period of time, but for the purpose of not generating newly a residual stress at the time of cooling, a cooling rate is made sufficiently low; a method in which reversely, for the purpose of positively utilizing a residual stress generated at the time of cooling to obtain a desired residual stress distribution, a cooling rate is made slightly fast; and so on. The former method involves such a drawback that since the time required for the cooling treatment becomes long, the productivity is conspicuously lowered, or contamination with impurities from the treatment environment is easily caused, or the like.
On the other hand, the foregoing method (2) is hereunder explained while taking the case of an optical system composed of two optical members as an example. In the case having such a distribution that not only birefringence of two optical members A and B each composed of a synthetic silica glass are identical, but the fast axis directions thereof are orthogonal to each other, since the fast axis of the optical member A and the fast axis of the optical member B are superimposed in the same direction, the birefringence of the two optical members are compensated each other, and the optical path integrated birefringence becomes zero.
In consequence, in order to reduce the optical path integrated birefringence of an optical system composed of plural optical members, it is effective to utilize the foregoing method (2) in addition to the foregoing method (1). In particular, in view of the fact that a requirement for reducing the birefringence of an individual optical member is reaching an extremely severe level from the standpoint of manufacture, it is expected that importance of the method (2) will increase in the future, and hence, it is necessary to control the fast axis direction of an optical member.
In view of the foregoing requirements, for example, Patent Documents 1 and 2 propose a manufacturing method in which the fast axis direction of a synthetic silica glass is controlled. In such a method, by controlling a concentration distribution of an OH group contained in the synthetic silica glass, a desired distribution of the fast axis direction is obtained.
On the other hand, in recent years, the following problems have been pointed out. All of the foregoing values of birefringence are a value regarding a shape, in general, a simple cylinder, during the shipment of materials by optical materials manufacturers; however, there is encountered such a problem that there may be the case where the birefringence measured in a state of being processed into a lens shape changes from a value before processing. In the case where an amount of change in the birefringence before and after processing is large, since the birefringence is different from that as previously assumed by an exposure apparatus manufacturer at the time of material purchase, a desired optical path integrated birefringence is not obtained. As causes of this change in the birefringence before and after processing, though an influence by a processing strain generated at the time of lens processing, or the like may be considered, a hypothesis that the change is caused by a stress distribution state in a thickness direction of the synthetic silica glass is the most influential at present.
The foregoing hypothesis is a theory that in the case where a portion with a locally high stress is included in a thickness direction, a large change is caused in the birefringence after processing. The conventional birefringence refers to a birefringence observed from a parallel direction to a principal optical axis direction, namely this is a value integrated in a thickness direction of the synthetic silica glass, and therefore, a stress state at each point in the thickness direction was unclear. For that reason, in order to reduce the change of the birefringence after lens shape processing, it is said to be necessary to reduce the stress at each point in the thickness direction of the synthetic silica glass. According to the measurement of a birefringence that is a means for measuring the stress, only a value integrated in the optical path of the measured light is obtained, and therefore, it is originally difficult to actually measure a local stress value at each three-dimensional point of the synthetic silica glass. However, as a substitution there for, a birefringence measured from a vertical direction to the principal optical axis direction, namely a birefringence in an off-axis direction, may be an index in view of supposing an amount of change in the birefringence after lens shape processing. It is said that the smaller this birefringence in the off-axis direction, the smaller the amount of change in the birefringence after processing is. From this fact, in optical members used for exposure apparatus in recent years, a reduction of the birefringence of a vertical component to the principal optical axis direction is also required.